1. Field of the Invention
The invention pertains to a method of operating a gas and steam turbine system, in which the heat contained in the expanded working medium of an associated gas turbine that can be operated with either gas or oil as its fuel is used to generate steam for an associated steam turbine connected in a water-steam loop, in which condensed steam from the steam turbine is supplied as condensate to the water-steam loop, and in which a partial flow is diverted from the water-steam loop for preheating purposes. The method is also directed to a gas and steam turbine system operating by the novel method.
2. Description of the Related Art
A gas and steam turbine system in which the heat contained in the expanded working medium from the gas turbine is used to generate steam for the steam turbine typically includes a waste heat steam generator. The heat transfer is effected by means of a number of heating surfaces, which are arranged in the form of pipes or pipe coils in waste heat steam generators. The waste heat steam generators in turn are connected into the water-steam loop of the steam turbine. The water-steam loop includes a plurality of pressure stages, for instance two or three of them, and each pressure stage has a preheater heating surface (economizer), an evaporator heating surface, and a superheater heating surface. With this kind of gas and steam turbine system, known for instance from EP 0 148 973 B1, a thermodynamic efficiency of about 50 to 55% is achieved, depending on the pressure conditions prevailing in the water-steam loop of the steam turbine.
A gas and steam turbine system is known from French patent application FR-A 2 551 181 in which the gas turbine can be operated with either gas or oil. A partial flow is diverted from the water-steam loop of that gas and steam turbine system for preheating purposes.
A gas and steam turbine system in which heating oil is contemplated as a backup for natural gas for only a short period of operation of the gas turbine, for instance for 100 to 500 h/a, the system is designed and optimized primarily for natural gas operation of the gas turbine. In order in the heating mode to raise the feedwater temperature at its entry into the waste heat steam generator without a complicated tapping of the turbine, the necessary heat can be drawn in various ways from the waste heat steam generator itself. One possibility is to bypass a usually provided condensate preheater completely or in part and to heat the condensate in a feedwater tank, connected to the water-steam loop, by delivering low-pressure steam. However, at low steam pressures, such a method requires a large-volume and sometimes multistage hot steam system in the feedwater tank, which for long heating times can threaten a degassing function that typically takes place in the feedwater tank.
To assure effective degassing of the condensate, the condensate temperature in the feedwater tank should as much as possible be kept within a temperature range between 130 and 160.degree. C., and the heating time of the condensate in the feedwater tank should be kept short. By way of example, this can be done by preheating the condensate via a steam-heated additional preheater. To furnish sufficient heat for this purpose, it is often necessary to draw hot water from a high-pressure economizer of the waste heat steam generator, in two- or three-pressure systems. However, especially in three-pressure systems, this has the disadvantage that a typically provided high-pressure feed pump can be varied in terms of its pumping quantity, and that the additional condensate preheater must be designed, in an especially uneconomical way, for both the high pressure and major pressure differences. In heating oil operation, throttling losses of the feed pump or of each feed pump also arise in a disadvantageous way. Moreover, drawing hot water from the high-pressure economizer reduces the high-pressure steam quantity by lowering the so-called high-pressure approach temperature, which in turn reduces the system efficiency.